1. Field of the Invention
The present invention relates to a resistance change type memory.
2. Description of the Related Art
Attention has been paid to a resistance change type memory such as MRAM (Magnetic Random Access Memory), ReRAM (Resistive Random Access Memory), and the like as a next generation non-volatile memory succeeding to a NAND type flash memory (refer to, for example, U.S. Pat. No. 6,256,223).
Data is written to the resistance change type memory by causing a write current to flow to, for example, a resistance change element as a memory cell and changing its state (resistance value). Further, data is read by causing a read current to flow to the resistance change element and detecting its resistance value. The value of the read current is set smaller than that of the write current.
There is disturb and retention as elements for evaluating the performance of a non-volatile memory.
The disturb means disturbance (erroneous writing) caused to a memory cell when data is read and written, the retention means a data-retention period of the memory cell.
Incidentally, when a memory cell is miniaturized, since a wiring resistance and the turned-on resistance of a transistor are increased, when a drive voltage has a predetermined value, the value of a write current, which can be caused to flow to the memory cell, is reduced.
However, the value of the write current affects the retention. That is, when the value of the write current is reduced, the data-retention period of a memory cell is generally shortened.
Further, a voltage applied to a memory cell when data is read affects reading sensitivity. Thus, to maintain sufficient reading sensitivity, the voltage applied to the memory cell when data is read, i.e., the value of a read current cannot be sufficiently reduced.
In this case, when the value of the write current is reduced as described above, since the voltage applied to the memory cell when data is written is reduced, the ratio between the voltage applied to the memory cell when data is read and the voltage applied to the memory cell when data is written is reduced.
Accordingly, when the value of the write current is reduced, read disturb is liable to occur.